1. Field of the Invention
This invention relates to a method for calibrating a sensor which is configured for checking value documents, e.g. in an apparatus for value document processing. Moreover, the invention relates to a corresponding sensor and to a corresponding value document processing apparatus.
2. Related Art
For checking value documents there are usually employed sensors with which the type of the value documents is determined and/or with which the value documents are checked for authenticity and/or their state. Such sensors are employed e.g. for checking bank notes, checks, identity cards, credit cards, check cards, tickets, vouchers and the like. The value documents are checked in an apparatus for value document processing which contains one or several different sensors, depending on the value document properties to be checked. The sensors are usually checked with regard to their correct operability at certain time intervals or because of a current event. For checking a sensor, it is first calibrated and subsequently adjusted, if necessary. The calibration is usually effected using calibration media which are supplied to the sensor and from which the sensor senses measurement signals. The calibration media can be configured for checking one or several properties of an individual sensor, or for checking several or all relevant properties of several or all relevant sensors of the apparatus for value document processing. For example, the calibration media used for calibrating bank-note sensors are paper sheets with known, predefined properties or also bank notes specially prepared for checking the sensors.
In some apparatuses for value document processing, the value documents are transported in the apparatus past the sensors employed for checking. To calibrate the sensors, there is transported past the sensors a calibration medium, instead of the value documents, whereby the sensors sense measured values of the calibration medium. The measured values are compared with target values which are associated with the calibration medium. If the measured values of the calibration medium deviate from the target values of the calibration medium, an adjustment of the relevant sensor is usually carried out, whereby the sensor is if possible so set that it delivers at least approximately the target values upon measurement of the calibration medium. The thus adjusted sensor is subsequently employed for checking value documents.
It is an object of the present invention to specify a method for calibrating a sensor configured for checking value documents which makes possible a precise calibration of the sensor.